A drilling fluid or mud is a specially designed fluid that is circulated through a wellbore as the wellbore is being drilled to facilitate the drilling operation. Drilling fluids facilitate the removal of drill cuttings from the wellbore, cool and lubricate the drill bit, aid in supporting the drill pipe and drill bit, and provide a hydrostatic head to maintain the integrity of the wellbore walls thereby minimizing the potential for well blowouts. Specific drilling fluid systems are selected to optimize a drilling operation in accordance with the characteristics of a particular geological formation.
Oil or synthetic-based muds, or invert emulsions, are normally used to drill swelling or sloughing shales, salt, gypsum, anhydrite or other evaporite formations, hydrogen sulfide-containing formations, and hot (greater than about 300° F.) holes, but may be used in other holes penetrating a subterranean formation as well. This class of drilling fluids typically contains oil or a synthetic oil or other synthetic material or synthetic fluid (“synthetic”) as the continuous phase and may also contain water which is dispersed in the continuous phase by emulsification so that there is no distinct layer of water in the fluid. The term “oil mud” or “oil or synthetic-based mud” typically means an invert oil mud emulsion or invert emulsion. An all oil mud simply comprises 100% oil by volume as the liquid phase; that is, there is no aqueous internal phase. An invert emulsion drilling fluid may commonly comprise between about 50:50 to 95:5 by volume oil phase to water phase.
Invert emulsions used in drilling typically comprise: a base oil or synthetic fluid for the external phase; a saline, aqueous solution for the internal phase (typically a solution comprising about 30% calcium chloride); and other agents or additives for suspension, fluid loss, density, oil-wetting, emulsification, filtration, and rheology control.
U.S. Pat. No. 6,006,831 (the '831 patent) generally discloses electrical well logging fluids and a method of using same. At column 12, starting at line 40, the '831 patent describes emulsifiers obtained by reaction of polyamines with a combination of fatty acids and basic acids such as maleic acids and fumaric acids. There is no disclosure, however, of the specific emulsifiers of the present invention derived from maleated tall oil fatty acid and higher polyamines or their effectiveness as emulsifiers in high temperature/high pressure applications.
U.S. Pat. No. 4,508,628 discloses fast drilling invert emulsion drilling fluids. The preferred polyamide emulsifiers are prepared by first reacting a polyamine with fatty acid in order to form a amidoamine and thereafter reacting the amidoamine intermediate with a dicarboxylic acid, preferably maleic or fumeric acid. It is apparent from the disclosure of this document, however, that the polyamine is limited to very short chain polyamines (Column #5 line 30). Additionally, the emulsifiers of this patent have to be dissolved in solvents to make it liquid at room temperature.
U.S. Pat. No. 3,169,113 discloses emulsifiers for water-in-oil acidic fracturing fluids. The disclosed emulsifier has two components:                (a) a 9-18 carbon monocarboxylic acid fatty acid salt of a partial amide of a polyalkyllene polyamine with 2-6 carbon alkylene groups and 3-5 amino nitrogens in which at least two amino groups are amidified with 9-18 carbon monocarboxylic fatty acids and wherein there is at least one nonamidified amino group forming a salt with the acid and,        (b) a polyamide of an alkylene polyamine with 2-6 carbon alkylene groups and 2-5 amino nitrogens and a 9-18 carbon monocarboxylic fatty acid.        
U.S. Pat. No. 4,501,672 discloses fluid loss reducing additives for oil-based working fluids. The additive is obtained by reacting 1 mole of dialkylene triamine with 2 moles of fatty acid. Example 1 specifically covers the bridging of diethylenetriamine dioleylamide with maleic anhydride.
WO 89/11516 discloses oil-based emulsifiers for drilling fluids that are the reaction product of one or two moles of an amide-amine or a hydroxylalkylamide with one to five moles of a dicarboxylic acid or an acid anhydride. Starting on page 5 of this document it is clear that the use of very short chain polyamines is contemplated.
U.S. Pat. No. 4,658,036 discloses an invert emulsion and a process of preparing same. The emulsifiers are prepared by reacting at least one tall oil fatty acid with acrylic acid, maleic anhydride, or fumaric acid, followed by reaction with diethylene triameen and at least one tall oil fatty acid in order to give the invert emulsifier.
With space at some well sites limited, such as on offshore platforms, and with increasing costs of transport of materials to a wellsite, there is industry-wide interest, on-going need for more efficient and concentrated drilling fluid additives and for drilling fluids which can be formulated and maintained with fewer additives in lesser amounts than commonly utilized with prior art drilling fluids.